The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Plasma arc torches, also known as electric arc torches, are commonly used for cutting, marking, gouging, and welding metal workpieces by directing a high energy plasma stream consisting of ionized gas particles toward the workpiece. In a typical plasma arc torch, the gas to be ionized is supplied to a distal end of the torch and flows past an electrode before exiting through an orifice in the tip, or nozzle, of the plasma arc torch. The electrode has a relatively negative potential and operates as a cathode. Conversely, the torch tip constitutes a relatively positive potential and operates as an anode during piloting. Further, the electrode is in a spaced relationship with the tip, thereby creating a gap, at the distal end of the torch. In operation, a pilot arc is created in the gap between the electrode and the tip, often referred to as the plasma arc chamber, wherein the pilot arc heats and subsequently ionizes the gas. The ionized gas is blown out of the torch and appears as a plasma stream that extends distally off the tip. As the distal end of the torch is moved to a position close to the workpiece, the arc jumps or transfers from the torch tip to the workpiece with the aid of a switching circuit activated by the power supply. Accordingly, the workpiece serves as the anode, and the plasma arc torch is operated in a “transferred arc” mode.
Manual plasma arc cutting torches can be operated in a variety of modes, including both standoff and drag cutting modes. With standoff cutting, an operator attempts to maintain a constant distance, or standoff, from the end of the tip or nozzle to the workpiece. If the distance becomes too large, the plasma stream diameter increases and thus the energy density decreases, which renders the plasma stream less effective in cutting the workpiece. Additionally, as the system is operating with a constant current, a higher voltage will be required with increasing distance, and if the distance becomes too large, the power supply will eventually shut off. On the other hand, if the distance between the tip and the workpiece becomes too small, molten metal may bridge the gap between the tip and the workpiece and result in double arcing. In this case, the tip becomes grounded and the flow of plasma is disturbed, which could lead to failure of the tip. Many plasma arc torches have employed a separate shield device to surround the tip and provide protection against the molten metal. The shield devices also provide some cooling to components at the front end of the plasma arc torch and can thus improve performance and life of the consumable components.
In the drag cutting mode, a front face of the tip is held in contact with the workpiece in order to maintain the constant standoff or distance between the tip and the workpiece. Drag cutting, however, results in mechanical abrasion of the tip from dragging, especially when the workpiece contains surface discontinuities. Heat from initiating the pilot arc is also detrimental to the tip since it is in contact with the workpiece, and molten metal often travels back up to the tip during operation. Heat that is transferred to the tip during operation is also detrimental due to the close proximity of the tip to the workpiece, and double arcing often occurs at the end of a cut as the tip is separated from the workpiece. These deleterious effects are only magnified when cutting at higher current levels, and as such, drag cutting is often limited to less than about 40 amps.